The present invention relates to detecting gaseous species in a mixture by light-emission spectroscopy.
In order to detect gaseous species, recourse has already been made to light-emission spectroscopy, in which use is made of the light radiation emitted by a plasma present in the gas mixture for analysis, the optical spectrum of said radiation emitted by the plasma is measured, and the optical spectrum is analyzed in order to deduce therefrom the presence of gaseous species in the mixture.
The conventional method used for the step of analyzing the optical spectrum consists in viewing the optical spectrum in real time and in comparing it with spectra published in scientific libraries and established for each gaseous species. The method relies on the fact that each gaseous species generates light radiation of spectrum that is characteristic when it reaches a level of excitation causing it to emit light. Scientific libraries thus contain the light-emission spectra for each gaseous species. Each spectrum is constituted by a curve plotting light intensity values as a function of wavelength over the wavelength range constituting light radiation, i.e. in the ultraviolet, in the visible spectrum, and in the infrared. Generally, the light-emission spectrum of a gaseous species is a jagged curve presenting a large number of peaks or “lines”. Each line is characterized by the wavelength and by the intensity of the light radiation and/or wavelength.
In known apparatuses, the light-emission spectrum is generally viewed by means of a computer which scans through the data issued by an optical spectrometer. Software associated with the spectrometer usually makes it possible to act on the integration time of the signal coming from the spectrometer, and thus on the intensity of the spectrum. The software may also act on the number of spectra to be averaged prior to display, thus making it possible to reduce noise. The software then allows the instantaneous light-emission spectrum to be viewed, and allows the variation in the amplitude of certain lines to be tracked, in order to deduce changes in the presence of a gas. The amplitude of a line at a defined wavelength generally makes it possible, when the gas is on its own, to track variation in the quantity of said gas that is present. The software also makes it possible to perform a certain number of mathematical operations such as subtracting spectra.
The light spectra of a gas mixture is generally constituted by the combination of the lines in the spectra that are specific to the various gaseous species present in the mixture.
It is sometimes possible, from the amplitudes of the lines of each specific spectrum, to deduce a measurement for the concentration of the corresponding gaseous species present in the mixture.
Measuring the concentrations of gaseous species in a mixture is quite easy and reliable when the gaseous species being sought out are easy to excite in the plasma. Such gaseous species that are easily excitable produce a light-emission spectrum having lines that can easily be seen and measured.
However, such a measurement of concentration becomes much more difficult for gaseous species that are more difficult to excite, particularly when those species are minority gaseous species, i.e. present in the mixture at a minority proportion only. Measuring the concentration of such poorly excitable or minority gaseous species in a mixture is possible at present only with measurement devices that are expensive, bulky, and difficult to operate, such as a mass spectrometer or a Fourrier transform infrared spectrometer (FTIR). For example, it is necessary to use such devices in order to measure traces of moisture in a vacuum in gas mixtures leaving a vacuum chamber in the semiconductor industry. Moisture is then present at a concentration of only a few thousands of parts per million (ppm). The cost of such measurements makes them economically unsuitable for use, particularly in methods of fabricating semiconductors.
The use of simpler spectroscopic measurement devices has not been envisaged for tracking traces of moisture. The difficulty comes in particular from the fact that the excitation of a gas in a plasma can vary strongly depending on the nature of the gas, and depending on whether the gas is alone or present in a gas mixture with other species.
For example, when considering moisture, the lines characteristic of the moisture to be observed (which moisture is present in only small quantity in the mixture) are poorly detectable or undetectable in the spectrum of the species that is present in a majority quantity, if it happens that that species is more easily excitable, as is the case for nitrogen. Tracking the lines characteristic of moisture, e.g. the hydrogen lines Hα, Hβ, and Hγ, the oxygen line at about 777.3 nanometers (nm), and the OH line at about 306.8 nm, for example, is practically impossible in a gas mixture where other gases, such as nitrogen, are more easily excitable and take all of the available energy.
That is why gaseous species that are in a minority and/or that are difficult to excite, such as moisture, and that are present in the gas mixtures of flows extracted from vacuum chambers in the semiconductor industry have not, as a general rule, been detected in the past by conventional methods of light-emission spectroscopy.